21. Although we produce carbon when we breathe, the carbon we produce is much less than produced by a car.
A. that B. it C. one D. the one
22. We should produce fewer things from raw materials, the supply is growing smaller and smaller.
A. whose B. of which C. where D. with which
23. I noticed that he was wearing shoes that did not match. He too hard to notice.
A. might be working B. can have worked
C. must have been working D. should have been worked
